This invention relates to new, water-soluble or water-dispersible resinous salts, to aqueous compositions containing them, and to surfaces coated with such compositions.
To obtain a corrosion-resistant coating for metal containers it is conventional to coat the metal surface with a crosslinkable resin formulation dissolved in an organic solvent and then to heat the coating to evaporate the solvent and to crosslink the resin. Crosslinking the coating converts it into a tough, adherent, flexible, and protective film. During heating, the solvent is usually evaporated into the atmosphere. Since organic solvents are relatively expensive, inflammable, and usually environmentally objectionable, there exists a need for coatings which may be applied using minimal proportions of such solvents, particularly useful being coating compositions which contain a high proportion of water.
We have now found that stable, aqueous compositions which cure to give coatings having excellent mechanical properties and chemical resistance may be prepared from new resinous salts. These salts are prepared from a phenol-terminated resin, either by thioalkylation with a mercaptocarboxylic acid and an aldehyde followed by at least partial neutralisation of the carboxylic acid group(s) introduced by the mercaptocarboxylic acid, or by sulphoalkylation with sulphurous acid (or a water-soluble salt thereof) and an aldehyde followed, if necessary, by at least partial neutralisation of the sulphonic acid group(s) introduced by sulphurous acid or its salt. They may be used with an aminoplast, a phenol-formaldehyde resin, or a blocked polyisocyanate as aqueous surface coating compositions. In certain circumstances the addition of such a coreactant is not, however, necessary.
The use of various reactions to obtain from phenols or epoxides water-soluble or water-dispersible resins for use as coatings has previously been described.
For example, British Pat. No. 1,254,528 discloses carboxyl group-containing polyethers said to be suitable for use in water-dilutable coating compositions and which are obtained by reaction of a glycidyl ether, a phenol-aldehyde condensate containing methylol groups, and a hydroxycarboxylic acid. The product is a polyether phenol-aldehyde resin etherified with the hydroxycarboxylic acid. Use of a mercaptocarboxylic acid to give a thioether-carboxylic acid is not envisaged. U.S. Pat. No. 4,153,586 describes producing a water-dispersible resin by reaction of a polyepoxide with a mercaptocarboxylic acid in the presence of a tin-containing catalyst.
Reaction between a phenol, an aldehyde, and an alkali metal bisulphite is also known (see, for example, Suter et al., J. Org. Chem., 1945, 10, 470-478) but, so far as we are aware, has not been carried out hitherto using a phenol-tipped advanced polyepoxide.